Light emitting diode and method for fabricating the same

ABSTRACT

A light emitting diode and a method for fabricating the same are provided. The light emitting diode includes: a transparent substrate; a semiconductor material layer formed on the top surface of a substrate with an active layer generating light; and a fluorescent layer formed on the back surface of the substrate with controlled varied thicknesses. The ratio of light whose wavelength is shifted while propagating through the fluorescent layer and the original light generated in the active layer can be controlled by adjusting the thickness of the fluorescent layer, to emit desirable homogeneous white light from the light emitting diode.

BACKGROUND OF THE INVENTION

This application claims priority from Korean Patent Application No.2002-52462, filed on Sep. 2, 2002, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

1. Field of the Invention

The present invention relates to a light emitting diode and a method forfabricating the same, and more particularly, to a light emitting diodecapable of emitting light with a homogeneous color profile and a methodfor fabricating the same.

2. Description of the Related Art

FIG. 1 is a sectional view of a conventional light emitting diodeemitting white light. Referring to FIG. 1, a blue-light emitting diode 2is located inside a lead frame 1, with wires 3 connected to the topsurface thereof via ohmic contact and to the lead frame 1 to supplyelectricity. The inner space of the lead frame 1 is filled with aphosphor 6 so that blue light emitted from the blue-light emitting diode2 is converted to red or green light or and then to white light by beingmixed up with the red or green light.

However, such a conventional blue-light emitting diode 2 cannot emithomogeneous white light profile and tends to emit light of wavelengthsdifferent from the wavelength of white light or conical light with ayellow or blue light ring.

Various solutions to this problem have been suggested. For example,according to a light emitting diode disposed in U.S. Pat. No. 5,813,753,as shown in FIG. 2, alight emitting diode 11 is provided in a cup-shapedheader 12. A mirror 13 is formed on the inner wall of the header 12 toreflect light emitted from the light emitting diode 11. The inner spaceof the header 12 is filled with a transparent material 15 containingphosphor grains 14 dispersed around the light emitting diode 11. A glassplate 16 is placed on the top of the header 12 to prevent light which isnot absorbed by the phosphor grains 14 from being emitted into the air.A low-wave pass (LWP) filter is further placed on the front side of thelight emitting diode 11 to pass short-wavelength light more efficientlythan long-wavelength light.

However, in manufacturing such conventional light emitting diodes, it isdifficult to control the amount of phosphor grains that is necessary toemit light of desired wavelength bands. A transparent materialcontaining phosphor grains should be deposited for individual lightemitting diodes. Accordingly, a great chromatic difference between theseparate light emitting diodes occurs, and the manufacturing timeincreases.

EP O 855 751 A2 discloses an organic/inorganic semiconductor lightemitting diode emitting red light and blue light that is manufactured byappropriately doping a green phosphor layer. However, it is difficult touniformly dope the phosphor layer to an appropriate ion concentration toobtain light of a uniform color profile.

SUMMARY OF THE INVENTION

The present invention provides a light emitting diode with a fluorescentlayer having controlled varied thicknesses, wherein the thickness of thefluorescent layer can be appropriately adjusted to enable the lightemitting diode to emit light of a desired wavelength band, and a simplemethod for fabricating the same.

According to an aspect of the present invention, there is provided alight emitting diode comprising: a substrate which transmits light; asemiconductor material layer formed on the top surface of a substratewith an active layer generating light; and a fluorescent layer formed onthe back surface of the substrate with controlled varied thicknesses.The substrate may have at least one etched hole formed by etching theback surface of the substrate to controlled varied thicknesses. Thefluorescent layer may be formed as dual layers with controlled variedthicknesses. It is preferable that the substrate is a sapphiresubstrate.

In an embodiment of the light emitting diode according to the presentinvention, the semiconductor material layer may comprise: a firstcompound semiconductor layer deposited on the top surface of thesubstrate; the active layer deposited on the top surface of the firstcompound semiconductor layer; and a second compound semiconductor layerdeposited on the top surface of the active layer. In this case, thefirst compound semiconductor layer may be an n-type doped or undopedGaN-based III-V nitride compound semiconductor layer. The secondcompound semiconductor layer may be a p-type doped GaN-based III-Vnitride compound semiconductor layer. The active layer may be an n-typedoped or undoped In_(x)Al_(y)Ga_(1-x-y)N compound semiconductor layerwhere 0≦x≦1, and x+y≦1.

In a light emitting diode according to the present invention, the activelayer generates blue light, and the fluorescent layer converts a portionof the blue light to yellow light to emit white light from the lightemitting diode. In this case, the fluorescent layer may be formed of afluorescent material including a garnet fluorescent material activatedwith cerium containing at least one element selected from the groupconsisting of yttrium, lutetium, scandium, lanthanum, gadolinium, andsamarium, and at least one element selected from the group consisting ofaluminum, gallium, and indium.

Alternatively, the active layer may generate UV light, and thefluorescent layer may convert the UV light to red, green, and blue lightby absorbing the UV light, to emit white light from the light emittingdiode. In this case, the fluorescent layer may be formed of afluorescent material containing a red phosphor selected from the groupconsisting of Y₂O₃Eu³⁺Bi³⁺ and Y₂O₂S, a green phosphor selected from thegroup consisting of(Ba_(1-x-y-z)Ca_(x)Sr_(y)Eu_(z))(Mg_(1-w)Zn_(w))Si₂O₇ and ZnS:Cu, and ablue phosphor selected from the group consisting of (Sr,Ba,Ca)₅(PO₄)₃CI:Eu²⁺) (SECA), BaMg₂Al₁₆O₂₇:Eu²⁺ (BAM), andBaMgAl₁₀O₁₇:Eu.

The present invention provides a light emitting diode with a fluorescentlayer having controlled varied thicknesses that can be implemented byetching the back surface of a substrate or by depositing a fluorescentmaterial on the back surface to controlled varied thicknesses. Accordingto the present invention, the emission ratio of original blue lightgenerated in an active layer and light absorbed by the fluorescent layerand converted to yellow light from the blue light can be controlled byappropriately adjusting the thickness of the fluorescent layer, to emithomogeneous white light from the light emitting diode. When the activelayer generates UV light, the emission ratio of the original UV lightand light absorbed by the fluorescent layer and converted to red, green,and blue light from the UV light can be controlled to emit homogeneouswhite light from the light emitting diode.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a sectional view of a conventional white-light emitting diode;

FIG. 2 is a sectional view of a light emitting diode disclosed in U.S.Pat. No. 5,813,753;

FIG. 3 is a sectional view of a light emitting diode according to afirst embodiment of the present invention;

FIG. 4 is a sectional view of a light emitting diode according to asecond embodiment of the present invention;

FIG. 5 is a sectional view of a light emitting diode according to athird embodiment of the present invention; FIG. 6 is a sectional view ofa light emitting diode according to a fourth embodiment of the presentinvention;

FIGS. 7A through 7D are sectional views illustrating a first embodimentof a method for manufacturing light emitting diodes according to thepresent invention;

FIGS. 8A through 8E are sectional views illustrating a second embodimentof the method for manufacturing light emitting diodes according to thepresent invention;

FIGS. 9A and 9B are photographs showing etched substrates of lightemitting diodes according to the first embodiment of the presentinvention; and

FIGS. 10A and 10B are photographs showing the back surface of thesubstrates of FIGS. 9A and 9B, respectively, with fluorescent layers.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of a light emitting diode and a method for fabricating thesame will be described in detail.

Referring to FIG. 3, which is a sectional view showing the structure ofa light emitting diode according to an embodiment of the presentinvention, a light emitting diode 50 includes a substrate 51, and afirst compound semiconductor layer 53, an active layer 57, and a secondcompound semiconductor layer 55, which are sequentially deposited on thetop surface of the substrate 51. An n-type electrode 54 is placed in astepped region of the first compound semiconductor layer 53, and ap-type electrode 52 is placed on the top surface of the second compoundsemiconductor layer 55, to supply electrons and holes into the activelayer 57.

The substrate 51 is made of a durable substance, mostly such assapphire. The back surface of the substrate 51 is etched to reduce thethickness of the substrate 51 in certain areas. Here, it is preferableto form an etched hole 56 a in the back surface of the substrate 51 byetching. In this case, the thickness of the substrate 51 is larger at aperipheral region 56 b than at the etched hole 56 a. The differentthicknesses of the substrate 51 enable a fluorescent layer 59 to bedeposited to controlled varied thicknesses onto the back surface of thesubstrate 51 by spin coating.

The first compound semiconductor layer 53 is a GaN-based III-V nitridesemiconductor layer, and preferably, is a direct transition type. In thecase of doping the first compound semiconductor with conductiveimpurities, a GaN layer is preferable for the first compoundsemiconductor layer 53. In either case, it is preferable that the firstcompound semiconductor layer 53 is formed of the same maternal as thesecond compound semiconductor layer 55. A first cladding layer (notshown) may be further formed on the top surface of the first compoundsemiconductor layer 53. Preferably, the first cladding layer may beformed of an n-AlGaN/GaN layer having a predetermined refractive index.However, the first cladding layer may be formed of a compoundsemiconductor layer different from the n-AlGaN/GaN layer.

The second compound semiconductor layer 55 is a GaN-based III-V nitridesemiconductor layer, and preferably, is a direct transition type dopedwith p-type conductive impurities, and most preferably, is a p-GaNlayer. In the case of undoping the second compound semiconductor layer55, a GaN layer or a AlGaN layer or InGaN layer containing Al or In,respectively, in a predetermined ratio may be used for the secondcompound semiconductor layer 55.

The active layer 57 is formed on the top surface of the first compoundsemiconductor layer 53. The active layer 57 is a material layer wherelight is generated by the recombination of electrons and carrier holes.Preferably, the active layer 57 is a GaN-based III-V nitridesemiconductor layer with a multiple quantum-well (MQW) structure. Morepreferably, the active layer 57 is formed of a In_(x)Al_(y)Ga_(1-x-y)N,where 0≦x≦1, 0≦y≦1, and x+y≦1, with a MQW structure.

First and second waveguide layers (not shown) may be further formed onand underneath the active layer 57, respectively, to amplify lightemitted from the active layer 57 and emit light from the LED withenhanced intensity. The first and second waveguide layers are formed ofa smaller refractive index material than the active layer 57, andpreferably, for example, a GaN-based III-V compound semiconductor layer.The first waveguide layer may be formed of a n-GaN layer, and the secondwaveguide layer may be formed of a p-GaN layer. The active layer 57 maybe formed of any material having a small threshold current value andstable transverse mode properties. Preferably, the active layer 57 isformed of an AlGaN layer containing Al in a predetermined ratio.

The second compound semiconductor layer 55 is formed on the top surfaceof the active layer 57. A second cladding layer (not shown) having asmaller refractive index than the second waveguide layer may beadditionally formed between the second compound semiconductor layer 55and the active layer 57. This second cladding layer is formed of ap-type compound semiconductor layer when the first cladding layer isformed of a n-type compound semiconductor layer, and is formed of ap-type compound semiconductor layer when the first cladding layer isformed of a p-type compound semiconductor layer. For example, when thefirst cladding layer is formed of a n-AlGaN/GaN layer, the secondcladding layer is formed of a p-AlGaN/GaN layer.

A pair of n-type electrodes 54 are laid on the two stepped regions ofthe first compound semiconductor layer 53, and the p-type electrode 52is laid on the top surface of the second compound semiconductor layer55, via which electrons and holes are injected into the first compoundsemiconductor layer 53 and the second compound semiconductor layer 55,respectively. The injected electrons and holes combine together anddisappear in the active layer 57 to oscillate light of ashort-wavelength band. The color of emitted light varies depending onthe wavelength band. The wavelength band of light is determined by theenergy width between the conduction band and valence band of thematerial used to form the light emitting diode 50.

III-V nitrides are commonly used to form semiconductor material layersemitting blue, green, and UV light. In the present invention,specifically, GaN-based semiconductor materials among III-V nitrides areused to enable the active layer 57 to generate blue light of awavelength of 420-470 nm or UV light and the generated blue light to betransmitted through a fluorescent layer 59 deposited on the back surfaceof the substrate 51. A portion of the generated blue light is absorbedin the fluorescent layer 59 and emitted as light of a differentwavelength band from the original blue light, for example, yellow light,and the non-absorbed blue light is emitted as blue light having theoriginal wavelength.

Various kinds of fluorescent materials may be selectively used dependingon the wavelength band of desired light to emit. When a light emittingdiode is formed of a nitride semiconductor material emitting blue light,as a fluorescent material capable of converting the blue light to yellowlight, a garnet fluorescent material activated with cerium (Ce)including at least one element selected from the group consisting ofyttrium (Y), lutetium (Lu), scandium (Sc), lanthanum (La), gadolinium(Gd), and samarium (Sm), and at least one element selected from thegroup consisting of aluminum (Al), gallium (Ga), and indium (In) may beused. To control the wavelength of emitted light, in a mixture of Ygroup, Al group, and garnet fluorescent materials, two kinds offluorescent materials selected from the Y group may be used together innon-equal amounts. For example, a portion of Y may be substituted by Gd.

In a light emitting diode with an active layer emitting blue light of awavelength of 420-470 nm, suitable fluorescent materials capable ofconverting the blue light to red light of a wavelength of 610-625 nminclude Y₂O₂S:Eu³⁺,Bi⁺; YVO₄:Eu³⁺,Bi³⁺; SrS:Eu²⁺; SrY₂S₄:Eu²⁺;CaLa₂S₄:Ce³⁺; (Ca, Sr)S:Eu²⁺ and the like. Suitable fluorescentmaterials capable of converting the blue light to green light of awavelength of 530-555 nm include YBO₃:Ce³⁺,Tb³⁺; BaMgAl₁₀O₁₇:Eu²⁺,Mn²⁺;(Sr,Ca,Ba)(Al,Ga)₂S₄:Eu²⁺ and the like. Any fluorescent materialemitting red light or green light may be used.

When a light emitting diode is formed of a nitride semiconductormaterial emitting UV light, a fluorescent material containing a redphosphor, such as Y₂O₃Eu³⁺Bi³⁺ and Y₂O₂S, a green phosphor, such as(Ba_(1-x-y-z)Ca_(x)Sr_(y)Eu_(z))(Mg_(1-w)Zn_(w))Si₂O₇ and ZnS:Cu, and ablue phosphor, such as (Sr, Ba,Ca)₅(PO₄)₃Cl:Eu²⁺) (SECA),BaMg₂Al₁₆O₂₇:Eu²⁺ (BAM), and BaMgAl₁₀O₁₇:Eu, is used for the fluorescentlayer 59 formed on the etched back surface of the substrate havingcontrolled varied thicknesses. In this case, UV light generated in theactive layer 57 is converted to red, green, and blue light whilepropagating through the fluorescent layer 59 and finally emitted fromthe light emitting diode as white light. In the present invention, asshown in FIG. 3, since the back surface of the substrate 51 has theetched hole 56 a, which is filled with the fluorescent layer 56, thethickness of the fluorescent layer 59 is larger at the etched hole 56 athan at the peripheral region 56 b. As a result, light generated in theactive layer 57 is absorbed more by the fluorescent material whilepropagating through the thicker region of the fluorescent layer 59 athan through the thinner region corresponding to the peripheral region56 b so that a larger amount of light whose wavelength band is shiftedcompared to the original light is emitted.

While the wavelength of emitted light is controlled using fluorescentmaterials in conventional light emitting diodes, the thickness of thefluorescent layer 57 is appropriately varied in the present invention inorder to emit light of a desired wavelength band. Alternatively,luminescence can be enhanced by changing the shape of the etched hole 56a of the substrate 51. For example, the sloping angle and the bottomcurvature of the etched hole 56 a may be varied in order to control theamount of light incident on the fluorescent layer 59 through thesubstrate.

FIG. 4 is a sectional view of a light emitting diode according to asecond embodiment of the present invention with a plurality of etchedholes.

Referring to FIG. 4, the back surface of the substrate 61 is etched toform a plurality of etched holes 66 a, and a fluorescent layer 69 isformed to fill over the etched holes 66 a in the substrate 61, so thatthe structure of a light emitting diode as shown in FIG. 4 is obtained.

Blue light or UV light generated in an active layer 67 of asemiconductor material layer 65 is transmitted through the substrate 62and enters the fluorescent layer 69. Since the fluorescent layer 69 hasa larger thickness at the etched holes 66 a than at peripheral regions66 b, light incident on the etched holes 66 a and propagating throughthe thicker region of the fluorescent layer 69 is likely to excite andabsorb more fluorescent grains present in the fluorescent layer 69,compared with light propagating through the peripheral regions 66 b. Inother words, the blue light or UV light generated in the active layer 67is highly likely to be converted to yellow light, or red, green and bluelight having a different wavelength from the original blue or UV lightwhile propagating through the thicker region of the fluorescent layer69, where the etched holes 66 a are formed. Also, light propagatingthrough the thinner region of the fluorescent layer 69, where theperipheral regions 66 b are formed, is highly likely to be emitted asthe original blue or UV light, without shifting in wavelength band.

The thickness of the fluorescent layer 69 can be adjusted to differentlevels by appropriately varying the number and the depth of etched holes66 a. As a result, light generated in the active region 67 of thesemiconductor material layer 65 is converted to light of wavelengthbands different from the original light while propagating through thefluorescent layer 69, so that homogeneous white light can be emittedfrom the light emitting diode.

In FIG. 4, reference numeral 62 denotes a p-type electrode, andreference numeral 64 denotes an n-type electrode. The material,properties, and function of the compound semiconductor layersconstituting the light emitting diode of FIG. 4 are the same as those ofthe light emitting diode according to the first embodiment describedabove.

FIG. 5 is a sectional view showing the structure of a light emittingdiode according to a third embodiment of the present invention.Referring to FIG. 5, a substrate 71 has a uniform thickness. A firstfluorescent layer 79 a is deposited on the back surface of the substrate71, and a second fluorescent layer 79 b is formed on a region of thefirst fluorescent layer 79 a. Accordingly, the entire fluorescent layer,including the first and second fluorescent layers 79 a and 79 b, hascontrolled varied thicknesses. Reference numeral 72 denotes a p-typeelectrode, reference numeral 74 denotes an n-type electrode, andreference numeral 75 denotes a semiconductor material layer.

FIG. 6 is a sectional view showing the structure of a light emittingdiode according to a fourth embodiment of the present invention.Referring to FIG. 6, a substrate 81 has a uniform thickness. A firstfluorescent layer 89 a is deposited on the back surface of the substrate81, and a plurality of second fluorescent layers 89 b are formed on thefirst fluorescent layer 89 a as stripes. Accordingly, the entirefluorescent layer, including the first and second fluorescent layers 89a and 89 b, has controlled varied thicknesses. Reference numeral 72denotes a p-type electrode, reference numeral 74 denotes an n-typeelectrode, and reference numeral 75 denotes a semiconductor materiallayer. Alternatively, a plurality of second fluorescent layers 89 a maybe formed as dots.

In the above-described third and fourth embodiments, the material,properties, and function of the compound semiconductor layerconstituting each of the light emitting diodes are the same as those ofthe light emitting diode according to the first embodiment of thepresent invention. The principles of emitting white light using thefluorescent layers 79 a (89 a) and 79 b (89 b) having controlled variedthicknesses are similar to those as in the first embodiment. Althoughthe fluorescent layer having controlled varied thicknesses isimplemented by etching the substrate in the light emitting diodesaccording to the first and second embodiments of the present invention,in the light emitting diodes according to the third and fourthembodiments of the present invention, the fluorescent layer havingcontrolled varied thicknesses is implemented using two separatefluorescent layers 79 a (89 a) and 79 b (89 b).

In the light emitting diodes according to the third and fourthembodiments of the present invention, when blue or UV light generated inthe active layer 77 (87) of the semiconductor material layer 75 (85)propagates through both of the first and second fluorescent layers 79 a(89 a) and 79 b (89 b), the blue or UV light is highly likely to beshifted in wavelength band and emitted as yellow light or red, green,and blue light, compared with blue or UV light propagating only throughthe first fluorescent layer 79 a (89 a). In other words, it is possibleto generate homogeneous white light by appropriately varying thethickness and the number of patterns constituting the second fluorescentlayer 79 b (89 b).

The light emitting diodes according to the first through fourthembodiments of the present invention described above are forillustrative purposes and, therefore, the shape and number of etchedholes and the thickness and the shape of the fluorescent layer may bevariously changed.

FIGS. 7A through 7D are sectional views illustrating a first embodimentof a method for fabricating light emitting diodes according to thepresent invention. Referring to FIG. 7A, a first compound semiconductorlayer 53, an active layer 57, and a second compound semiconductor layer55 are deposited in sequence on the top surface of a substrate 51, andthe first compound semiconductor layer 53 is patterned byphotolithography to form a step in the first compound semiconductorlayer 53. N-type electrodes 54 are laid on the patterned surface of thefirst compound semiconductor layer 53 as stripes, and p-type electrodes52 are laid on the top surface of the second compound semiconductorlayer 55.

Referring to FIG. 7B, etched holes 56 b, which are filled with afluorescent layer 57 later, are formed in the back surface of thesubstrate 51 by dry etching. Prior to etching the substrate 51, the backsurface of the substrate 51 is processed by grinding, lapping, orpolishing. A mask layer (not shown) is formed on the back surface of thesubstrate 51 and patterned into a mask pattern corresponding to theetched holes 56 a. The back surface of the substrate 51 is etched usingat least one gas selected from the group consisting of Cl₂, BCl₃, Ar,O₂, and HBr, with the mask pattern serving as an etch mask, so that theetched holes 56 are formed as shown in FIG. 7B.

FIGS. 9A and 9B show etched holes formed in sapphire substrates byetching. Etched holes 56 c of FIG. 9A are relatively wide and shallow,and etched holes 56 d of FIG. 9B are relatively narrow and deep. Thedepth and width of the etched holes 56 c (56 d) are determined inconsideration of the thickness of the fluorescent layer depositedtherein. For example, the back surface of a sapphire substrate is etchedsuch that the resulting etched holes 56 c (56 d) have a depth of about50 μm and a width of 250-500 μm.

Referring to FIG. 7C, after the etched holes 56 a have been formed, afluorescent material is applied to the etched holes 56 a and theperipheral regions 56 b by disposing or spin coating to form thefluorescent layer 59, as shown in FIG. 6C. As a result, a light emittingdiode structure 58 is formed. According to the present invention, thefluorescent layer 59 can be uniformly deposited over the entiresubstrate 51 through a single process. Therefore, the overall processfor fabricating light emitting layer diodes is simple.

FIGS. 10A and 10B are photographs of light emitting diodes withyttrium-aluminum-garnet (YAG) fluorescent layers 59 c and 59 d formed byapplying a fluorescent material to fill the etched holes 56 c and 56 din the back surface of the substrates of FIGS. 9A and 9B, respectively.In FIGS. 10A and 10B, the YAG fluorescent layers 59 c and 59 d in theetched holes appear white.

Finally, as shown in FIG. 7D, the light emitting diode structure 58 iscut, at connection regions with two adjacent n-type electrodes 54, intoa plurality of light emitting diodes 50.

FIGS. 8A through 8E are sectional views illustrating a second embodimentof the method for fabricating light emitting diodes according to anembodiment of the present invention. Referring to FIG. 8A, a firstcompound semiconductor layer 73, an active layer 77, and a secondcompound semiconductor layer 75 are deposited in sequence on the topsurface of a substrate 71, and the first compound semiconductor layer 73is patterned by photolithography to form a step in the first compoundsemiconductor layer 73. N-type electrodes 74 are laid on the patternedsurface of the first compound semiconductor layer 73 as stripes, andp-type electrodes 72 are laid on the top surface of the second compoundsemiconductor layer 75.

Referring to FIG. 8B, a fluorescent material is applied to the backsurface of the substrate 71 by disposing or spin coating to form a firstfluorescent layer 79 a. Next, as shown in FIG. 8C, a mask 76 having apredetermined pattern is placed on the first fluorescent layer 79 a, andthe fluorescent material is applied to form a second fluorescent layer79 b on the first fluorescent layer 79 a. As a result, a light emittingdiode structure 78 with a fluorescent layer, including the first andsecond fluorescent layers 79 a and 79 b, having controlled variedthicknesses is formed, as shown in FIG. 8D. Finally, as shown in FIG.8E, the light emitting diode structure 78 is cut into a plurality oflight emitting diodes 50, thereby completing the fabrication of desiredlight emitting diodes 50.

A light emitting diode according to the present invention is fabricatedwith a fluorescent layer having controlled varied thicknesses, whereinthe fluorescent layer having controlled varied thicknesses may be formedby etching a substrate to controlled varied thicknesses and applying afluorescent material to the etched surface of the substrate.Alternatively, the fluorescent layer having controlled variedthicknesses may be formed by depositing a fluorescent material tocontrolled varied thicknesses on a substrate having a uniform thickness.In the light emitting diode according to the present invention, lightgenerated in an active layer is shifted in wavelength while passingthrough the fluorescent layer having controlled varied thicknesses. Theratio of light whose wavelength band is shifted while propagatingthrough the fluorescent layer and the original light generated in theactive layer can be controlled by varying the thickness of thefluorescent layer so that desired homogeneous white light can be emittedfrom the light emitting diode according to the present invention. Amethod for fabricating light emitting diodes according to the presentinvention, which involves simple processes, for example, etching theback surface of a substrate or applying a fluorescent material over thesubstrate by disposing or spin coating, is suitable for mass production.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1-37. (canceled)
 38. A light emitting device comprising: a lighttransmitting substrate having a top surface, a back surface opposite thetop surface and a side surface between the top surface and the backsurface; a first compound semiconductor layer, an active layergenerating first light and a second compound semiconductor layersequentially formed on the top surface of the substrate, a portion ofthe first semiconductor layer being exposed and most of first lightbeing emitted through the back surface of the substrate; a firstelectrode disposed on the exposed portion of the first semiconductorlayer and a second electrode disposed on a top surface of the secondsemiconductor layer, respectively; and a wavelength converting layerhaving first and second fluorescent layers sequentially formed on theback surface of the substrate and converting a portion of the firstlight to second light having a different wavelength from the firstlight, wherein the first fluorescent layer has a different thicknessesfrom the second fluorescent layer.
 39. The light emitting device ofclaim 38, wherein the first and second fluorescent layers contain aninorganic fluorescent material.
 40. The light emitting device of claim38, wherein the first fluorescent layer contains a first inorganicfluorescent material and the second fluorescent layer contains a secondinorganic fluorescent material being different from the first inorganicfluorescent material.
 41. The light emitting device of claim 38, whereineach of the first and second fluorescent layers has a uniform thickness.42. The light emitting device of claim 38, wherein the substrate is asapphire substrate.
 43. The light emitting device of claim 42, whereinthe first compound semiconductor layer is an n-type doped or undopedGaN-based III-V nitride compound semiconductor layer.
 44. The lightemitting device of claim 42, wherein the second compound semiconductorlayer is a p-type doped GaN-based III-V nitride compound semiconductorlayer.
 45. The light emitting device of claim 42, wherein the activelayer is an n-type doped or undoped InAlGaN compound semiconductor layerwhere 0≦x≦1, 0≦y≦1, and x+y≦1.
 46. The light emitting device of claim38, wherein a first light is blue light of a wavelength of 420˜470 nm,and the second light includes yellow light.
 47. The light emittingdevice of claim 46, wherein the first or second fluorescent layer ismade of an inorganic fluorescent material including a garnet fluorescentmaterial activated with cerium containing at least one element selectedfrom the group consisting of yttrium, lutetium, scandium, lanthanum,gadolinium, and samarium, and at least one element selected from thegroup consisting of aluminum, gallium, and indium.
 48. The lightemitting device of claim 46, wherein the second light further includesred or green light, wherein one of the first and second fluorescentlayers converts blue light to yellow light and the other of the firstand second fluorescent layers converts blue light to red or green light.49. The light emitting device of claim 48, wherein the red light has apeak wavelength of 610˜625 nm and/or the green light has a peakwavelength of 530˜555 nm.
 50. The light emitting device of claim 46,wherein one of the first and second fluorescent layers is made of aninorganic fluorescent material including a garnet fluorescent materialactivated with cerium containing at least one element selected from thegroup consisting of yttrium, lutetium, scandium, lanthanum, gadolinium,and samarium, and at least one element selected from the groupconsisting of aluminum, gallium, and indium.
 51. The light emittingdevice of claim 50, wherein the other of the first and secondfluorescent layers is made of at least one of red and green fluorescentmaterials, the red fluorescent material includes at least onefluorescent material of Y₂O₂S:Eu³⁺,Bi³⁺, YVO₄:Eu³⁺,Bi³⁺, SrS:Eu²⁺,SrY₂S₄:Eu²⁺, CaLa₂S₄:Ce³⁺, (Ca, Sr)S:Eu²⁺and the green fluorescentmaterial includes at least one fluorescent material of YBO₃:Ce³⁺, Tb³⁺,BaMgAl₁₀O₁₇:Eu²⁺,Mn²⁺, (Sr, Ba, Ca)(Al, Ga)₂S₄:Eu²⁺.
 52. The lightemitting device of claim 38, wherein the first light is blue lighthaving a wavelength of 420˜470 nm, and the second light includes red andgreen light.
 53. The light emitting device of claim 50, wherein one ofthe first and second fluorescent layers converts blue light to red lightand the other of the first and second fluorescent layers converts bluelight to green light.
 54. The light emitting device of claim 38, whereinthe first light is UV light, and the second light includes red, greenand blue light.
 55. The light emitting device of claim 38, wherein thewavelength converting layer has a side surface being substantiallycoplanar with the side surface of the substrate.
 56. A light emittingdevice comprising: a light transmitting substrate having a top surface,a back surface opposite the top surface and a side surface between thetop surface and the back surface; a flip chip type LED disposed on thetop surface of the substrate, the flip chip type LED emitting a firstlight, most of which is emitted through the back surface of thesubstrate; and a wavelength converting layer having first and secondfluorescent layers sequentially formed on the back surface of thesubstrate and converting a portion of the first light to second lighthaving a different wavelength from the first light, wherein the firstfluorescent layer has a different thicknesses from the secondfluorescent layer.
 57. The light emitting device of claim 56, whereinthe flip chip type LED device comprises: a first compound semiconductorlayer, an active layer generating first light and a second compoundsemiconductor layer sequentially formed on the top surface of thesubstrate, a portion of the first semiconductor layer being exposed; anda first electrode disposed on the exposed portion of the firstsemiconductor layer and a second electrode disposed on a top surface ofthe second semiconductor layer, respectively.
 58. The light emittingdevice of claim 56, wherein the first fluorescent layer converts aportion of the first light to yellow light.
 59. The light emittingdevice of claim 56, wherein the second fluorescent layer converts aportion of the first light to red or green light.
 60. The light emittingdevice of claim 56, wherein the second fluorescent layer converts aportion of the first light to green light and red light.
 61. The lightemitting device of claim 56, wherein the first fluorescent layerconverts a portion of the first light to green light and the secondfluorescent layer converts a portion of the first light to red light.62. The light emitting device of claim 56, wherein the first or thesecond fluorescent layer is made of an inorganic fluorescent materialincluding a garnet fluorescent material activated with cerium containingat least one element selected from the group consisting of yttrium,lutetium, scandium, lanthanum, gadolinium, and samarium, and at leastone element selected from the group consisting of aluminum, gallium, andindium.
 63. The light emitting device of claim 56, wherein the first orthe second fluorescent layer is made of yttrium-aluminum-garnet (YAG)fluorescent material.